I know it is tricky question, but my friend is 20 years old and would love to start running GH peptides. Is that ok or is it's risky and he might mess something up with his natural hormones?
You can use it at any age (18++), but you would see more benefit as you get older.
In my opinion if you don't have an actual need for them (for example healing/skin issues) then personally, I'd wait until I am older to experience it's full effect. There'll be a benefit always, but most likely not too visible if you're in your twenties..
If you are under let's say.. 25-28, I would say do not bother except if, as previously mentioned, you've got a certain injury that needs to be healed.